FIGS. 7(a) to 7(a) show cross-sectional views of the major production process steps of a method of producing a semiconductor device according to the prior art.
First of all, as shown in FIG. 7(a), an active layer 2 is produced at a desired position on a semi insulating GaAs substrate 1 by ion implantation. Thereafter, as shown in FIG. 7(b), a refractory metal silicide layer, such as WSix layer 3 is plated on the entire surface of semi-insulating GaAs substrate 1 by sputtering or deposition another deposition process and a gate pattern comprising photoresist film 6 is formed on a portion where a gate is to be produced.
Next, as shown in FIG. 7(c), the WSix layer 3 is selectively etched by reactive ion etching (hereinafter referred to as RIE) using CF.sub.4 +O.sub.2 or SF.sub.6 gas using the photoresist film 6 as a mask. Then, ion implantation using the selectively etched WSix layer 3, that is, a refractory metal gate, as a mask and annealing are carried out to produce a high concentration doping layer 4 (FIG. 7(d)). Thereafter, as shown in figure 7(e), an insulating film (SiO2 film) 5 is deposited on the entire surface of semi-insulating GaAs substrate 1 in a plasma CVD apparatus. As shown in FIG. 7(f), a photoresist film 9 is deposited thereon to flatten the surface.
Thereafter, the WSix layer 3 is exposed by etching the photoresist film 9 and the insulating film 5 at equal rates (FIG. 7(g)). Thereafter, as shown in FIG. 7(h), the photoresist film 7 is formed into a pattern, and the multiple layer, low resistance metal film 8 such as Ti/Mo/Au, is deposited and lifted off. Thereafter, as shown in FIG. 7(i), a source and drain electrode 10 are deposited.
In the production method of a semiconductor device according to the prior art, however, it is necessary to expose the WSix film surface uniformly over the entire surface of wafer in the process of exposing the top portion of the refractory metal gate electrode (WSix film) 3. To achieve that result, a high uniformity in the insulating film thickness and in the planarity of the surface of photoresist film are required, thereby resulting in reduction in the yield.
Furthermore, the growth of the insulating film, depositing of the photoresist film, or etching back of the photoresist film have to be conducted by separate apparatus, thereby resulting in problems in the work efficiency.